He writes a letter after the Weasleys free him.  He writes it to Dumbledore to explain, to confirm that there were reasons that he had asked to stay at the school for the summer.

He sucks up his pride and tells him about his so-called bedroom with the seven locks and the cat flap and the darkness.  He tells him about his meals, such as they were and the utter loathing he saw on each of his guardians’ faces whenever they had to interact with him.

He throws up after sending the letter, his stomach tied in knots by his admissions, and from the very real fear that Dumbledore might go and talk to the Dursleys and believe them over him.

As it happens, he need not have worried about that.  

'My dear boy,

I don’t see what you hope to gain from exaggerating such tales about your family.  However . . .'

Whatever the old man says after that is lost to his accidental magic as the parchment suddenly catches fire and burns away to nothing in seconds.

He trembles violently, almost as though his magic knows that something has changed.  

Or, that something will change.